Protective circuits



Jan. 7, 1941. J PQCH 2,227,603

' PROTECTIVE CIRCUITS OriginalFiled May 26, 1956v Q Q o 8 Q g 3 \1; J

a, N Z4 25 Zmvcntor Waldemar J Pooh I Gttonleg Patented Jan. 7, 1941 PATENT OFFICE PROTECTIVE cmcm'rs Waldemar J. Poch, Oollingswood, N. J., assignor to Radio Corporation of Delaware of America, a corporation Original application'May 26, 1936, Serial No.

81,809, now Patent No. 2,178,764, dated Novemher 7, 1939. Divided and this application March 31, 1939, Serial No. 265,188

3 Claims.

My invention relates to protective circuits for thermionic devices and particularly to protec-- tive circuits for cathode ray tubes used in television receivers.

This application is a division of my application Serial No. 81,809, filed May 26, 1936,. entitled Protective circuits and assigned to the Radio Corporation of America, which issued November 7, 1939, as Patent No. 2,178,764.

In a preferred type of television receiver employing a cathode ray tube, the control grid of this tube is connected by a direct current connection to the plate of the last amplifier tube whereby the last amplifier stage will amplify direct current so that automatic control of the picture background may be obtained. In such a circuit the negative bias for the control grid of the cathode ray tube is obtained from the voltage drop in the plate circuit of the above mentioned amplifier tube. Since the cathode of the amplifier tube ordinarily is of the indirectly heated or equi-potential type there is an appreciable interval after the power has been supplied to the receiver before plate current flows through the amplifier tube and its plate circuit. During this warm up period, the cathode ray, tube control grid is not maintained at a negative potential unless special precautions are taken, and the cathode ray tube may be injured as a result.

Accordingly, it is an object of my invention to provide an improved circuit whereby injury to a cathode ray tube or the like during the warm up period is prevented. I i

More specifically, an object of my invention is to provide in an amplifier of the direct current type a protective circuit which will maintain the control grid of a thermionic device negative immediately after power has been applied to the circuit.

In practicing my invention, I employ a circuit of the general type described and claimed in Patent No. 2,092,373, issued September 7, 1937, in the name of Ralph S. Holmes and assigned to the Radio Corporation of America. As in the circuit described inthe Holmes application, I include a thermionic device in the grid-cathode circuit of the cathode ray tube to interrupt this circuit, so far as the flow of direct current is concerned, during the warm up period. In accordance with my invention, however, I so designed the circuit that not only does the control grid of the cathode ray tube never go positive, but it is biased negatively at all times that power is applied to the circuit.

My invention will be more fully understood by referring to the accompanying drawing in which the single figure is a circuit diagram of one embodiment of my invention.

Referring to the drawing, my invention is shown applied to a television receiver including a cathode ray tube I and an amplifier tube 2. The cathode ray tube is of conventional design comprising an indirectly heated cathode 3, a control grid 4, a first anode 6, a second anode and a fluorescent screen 8. The amplifier tube 2 is of a well known type including an indirectly heated cathode II, a control grid |2, a plate l3, and a diode plate M which has for its cathode a portion of the main cathode II.

The input circuit of the amplifier tube 2 in- 15 cludes a grid condenser l6 and a grid resistor H which are so adjusted that incoming synchronizing impulses produce a biasing potential across the grid resistor. The magnitude of this negative bias varies in accordance with the varying height 20 of incoming synchronizing impulses whereby the flowof direct current through the plate circuit of the amplifier tube 2 is a measure of the picture background. The varying voltage drop caused .by this change in direct current flow ap- 25 pears across the plate resistor l8 and is applied to the control grid 4 of the cathode ray tube by a direct current connection I9. An automatic background control circuit operating in this way is described and claimed in application Serial No. 30 718,192, filed March 30, 1934, in the name of Waldemar J. Pooh and assigned to the Radio Corporation of America.

' Voltage for the plate of the amplifier tube 2 is supplied from any suitable direct current source 35 such as the voltage divider 2| which is connected at one end to ground and to the negative terminal of a rectifier-filter unit (not shown) and connected at the other end to the positive terminal of this rectifier-filter unit.

A shunt circuit is connected across the voltage divider 2|, this shunt circuit including a potentiometer resistor 22 connected at one end to the positive end of the voltage divider 2| and connected at the other end through a resistor 25 and 45 a conductor 23 to the diode plate M. The shunt circuit is completed through the diode |l--|4 to ground and through ground to the negative end of the voltage divider 2|. The cathode 3 of the cathode ray tube is connected through a variable 50 tap 24 to a point on the shunt resistor 22. The cathode 3 is also provided with an alternating current connection to ground through a by-pass' condenser 26.

In this embodiment of my invention the plate 55 [3 of the amplifier tube 2 is preferably connected to the most positive point on the voltage divider 2| whereby the control grid 4 of the cathode ray tube would be without negative bias during the warm up period if the teachings of the prior art were followed.

I overcome this difficulty by connecting a resistor 3| between the direct current connection l9 and ground whereby, during the warm up period, current flows from the positive terminal of the voltage divider 2|, through the plate resistor l8 and the additional resistor 31 to ground, and through ground to the negative end of the voltage divider 2 I. Since at this time there is no current flow through the diode section of the amplifier tube 2, the cathode 3 of the cathode ray tube is at the same potential as the lower end of It will be apparent, therethe peaking coil 21. fore, that the flow of current through the plate resistor I8 and peaking coil 21 during the warm up period applies a negative potential to the control grid 4 whereby damage to the cathode ray tube is prevented.

As soon as the amplifier tube 2. warms up, plate current flows through the plate resistor I8 and peaking coil 21 to apply a further negative bias to the control grid 4 of the cathode ray tube and current flows through the shunt resistor 22 and diode tending to impress a positive voltage upon the control grid 4; thus by adjusting the variable tap 24 on the resistor 22 the negative bias on the control grid 4 may be adjusted to the desired value.

The resistor 25 is included between the potentiometer 22 and the diode plate M in order that the slider 24 cannot be moved to a position where a positive voltage is applied to the grid 4. In the circuit illustrated, the grid 4 is about 10 volts negative even though the slider 24 is in its most positive position.

It may be noted that, when the protective diode is in the same envelope with the amplifier electrodes, there is no danger of injuring the cathode ray tube by removing the last amplifier tube while the power is on since this breaks both the amplifier plate circuit and the shunt potentiometer circuit. 7

-The resistance of certain circuit elements has been-indicated on the drawing in ohms. These values are given merely by way of example and may be varied within wide limits.

I claim as my invention:

1. Electrical apparatus comprising. an amplifier tube having a cathode and a plate, an electric discharge tube having a cathode and a control grid, a direct conductive connection between said plate and said control grid, 2. source of direct current having positive and negative terminals, a circuit inshunt to said source, said circuit including a resistor having one end connected to said positive terminal and having in series therewith a device through which current flows readily only when heat is applied thereto, said device having at least as much thermal time lag as the cathode of said amplifier, a direct current connection between the cathode of said electric discharge tube and a point on said resistor, a connection between the cathode of said amplifier tube and said negative terminal, aplate resistor connected between the plate of said amplifier tube and a point on said source which is positive with respect to said negative terminal, and a protective resistor connected between the plate and the cathode of said amplifier.

2. In combination, a cathode ray tube having a control grid and a cathode which is electronemissive when heated, an amplifier tube having an anode and a cathode which is electron-emissive when heated, a direct conductive connection between said anode and said control grid, a voltage divider having a positive terminal and a negative terminal, a circuit in shunt to said voltage divider, said circuit includng a potentiometer resistor having one end connected to said positive terminal and having in series therewith a device through which current flows readily only when heat is applied thereto, said device having at least as much thermal'time lag as the cathode of said amplifier, a direct current connection between the cathode of saidcathode ray tube and a point on said potentiometer resistor, a connection between said negative terminal and the cathode of said amplifier tube, a resistor connected between the anode of said amplifier tube and a point on said voltage divider which is positive with respect to said negative terminal, and a protective resistor connected between the anode of said amplifier tube and a point on said Voltage divider which is negative with respect to said last-named point.

3. In combination, acathode ray tube having a control grid, and a cathode which is electronemissive when heated, an amplifier tube having an anode and a cathode which is electronemissive when heated, an auxiliary diode plate associated with said cathode, a direct conductive connection between said anode and saidcontrol grid, a voltage divider having a positive terminal and a negative terminal,.a circuit in shunt to. said voltage divider, said circuit including a potentiometer resistor having one end connected to said positive terminal. and having the other end connectedv to said diode plate, a direct current connection. between the cathode of said cathode ray tube and a point onsaid potentiometer resistor, a connection between the cathode. of said amplifier tube and said negative terminal, a resistor connected between the anode of said amplifier tube and a point on said voltage divider which is positive with respect to said negative terminal, and a protective resistlor connected between the anode of said amplifier tube and a point on said voltage divider which is negative with respect to said last-named point.

WALDEMAR J. POCH. 

